The total solar eclipse passing over the United States on August 21 is going to be disruptive. Authorities are predicting huge traffic jams, strained cellphone networks, and insufficient bathrooms for the masses driving to the center of the show.
But there’s another disruption that will be brought on by the eclipse: power.
Since the last total solar eclipse passed over part of the US in 1979, we’ve grown a lot more dependent on solar to electrify our homes and businesses. According to the Solar Energy Industries Association, solar energy has grown by an average of 68 percent per year in the past decade. The country now has about 45 gigawatts of solar capacity installed, with 260,000 Americans employed in the industry.
The solar eclipse will significantly diminish that capacity for a couple of hours on August 21, especially in California and North Carolina.
“Our solar plants are going to lose over half of their ability to generate electricity during the two to two and a half hours that the eclipse will be impacting our area,” says Steven Greenlee, spokesperson for the California Independent System Operator, or CAISO, one of the largest independent grid operators in the world.
During the eclipse, we can expect the moon to completely (and partially, in the areas beyond the totality) obscure the sun’s rays across the US, blocking the sunlight that powers the solar panels. Unlike on a cloudy day, this loss will cause a rapid decline and rebound of solar power that grid operators will have to skillfully manage.
The federal Energy Information Administration expects 1,900 utility-scale solar photovoltaic (PV) power plants in all will be affected.
The North American Electric Reliability Corporation (NERC) says it doesn’t expect the eclipse to create reliability issues for the bulk power system. Still, the eclipsewill test the reliability of solar generation on a national scale, and challenge the preparedness of operators like CAISO.
Ultimately, the eclipse is a reminder of how important solar energy has become to our power generation, and that with preparation, this system can work even when its energy source — the sun — is blotted out.
Why the solar eclipse is a big deal for solar and grid operators
Thanks to highly supportive solar policies and an abundance of sunny days, California is a leading solar market in the US. The state is currently responsible for nearly half of the nation’s solar capacity, according to the EIA. This year alone, more than 4.8 million homes were powered by solar facilities — ranging from large-scale power plants to individual PV panels.
The upcoming solar eclipse will darken California skies from approximately 9:30 to 11:30 am PST, dimming solar radiation by 70 percent. And though the line of totality will not pass through California, people in the solar energy industry there are prepping for the impact.
To maintain grid reliability, solar facilities will have to use a complex combination of other sources of power generation. Natural gas plants and hydro plants will likely be the go-to alternatives to fill the demand gap.
Greenlee says CAISO’s greatest challenge will be managing both timing and ramp rates, or changes in energy output. In an eclipse, the sun’s energy will ramp down much faster than it would even on a cloudy day. “While solar is ramping down, we have to ramp up the other generation,” he explains. “But once the eclipse is over and the sun starts coming back, we have to ramp down the other generation in order to keep supply balanced with demand.”
California utilities say they’re ready for the eclipse
Solar facilities have long been anticipating this eclipse, mapping out step-by-step demand management for the day of and arranging substitute energy sources to dispatch depending on various demand scenarios. Thanks to the unusually wet winter in California, hydroelectricity is abundant this year, says Greenlee.
In the past few months, CAISO — which manages 80 percent of California’s electric flow — has been busy seeking advice from German solar facilities.
During a 2015 solar eclipse that passed over Europe, 80 percent of Germany’s sunlight was cut off. For a country whose electricity is 40 percent powered by solar, it was hit hard. But despite the dramatic seesawing of solar production, the eclipse came and went without major disturbance. As I wrote for NOVA Next, Germany stabilized its grid and electricity supply by dialing up its fossil fuel, nuclear, and hydro power, while also asking four energy-hogging aluminum smelters to dial down their power use temporarily.
The California Public Utilities Commission (CPUC) is similarly encouraging user reduction from 9 to 11 am on the day of the eclipse. In a recent press release, CPUC stated that consumer cooperation would mean CPUC wouldn’t have to rely on inefficient, expensive, and often fossil fuel–dependent peakers (or “last resort” power plants) to make up for the imbalance of supply and demand.
However, Michael Picker, president of CPUC, doesn’t believe the eclipse will have a serious impact on CPUC’s ability to provide service on the morning of August 21. “We have been planning and thinking about it for a while to come, so I don’t think there will be any surprise,” he says. “I think this is well within the range of the many scenarios that the Independent System Operator actually models.”
Thankfully, the time at which the eclipse is expected to hit California is not the time of peak electricity use. Usually, peak usage curves from 4 to 9 pm. Picker hopes this eclipse will instead start a conversation about what it means have a grid that’s heavily reliant on fitful natural resources.
California isn’t the only state anticipating the eclipse’s impact on its solar capacity, though. The eclipse will cut through a total of 14 states, many of which have also become dependent on solar generation. At the time of highest impact — 1:40 EDT (10:30 PDT) when the eclipse is passing through Wyoming — the MDA solar forecasting system predicts solar generation potential across the country will decrease to less than 50 percent of capacity.
Georgia, Tennessee, Missouri, Idaho, and Oregon in the path of totality have all added a sizable amount of solar into their electricity markets. According to the EIA, Oregon has 17 utility-scale solar PV generators, mostly in the eastern part of the state, that will be impacted by the eclipse.
North Carolina — second only to California in installed capacity — powered 371,000 homes with solar energy this past year, and it is expecting a drop in solar output from 2.5 GW to 0.2 GW for 1.5 hours around the time of the eclipse. In a blog post, Duke Energy, which handles more than three-quarters of the nearly 3.3 GW of solar power generated in North Carolina, said that “operators will have natural gas plants ready to step in during the eclipse. In addition to replacing the lost energy with a flexible fuel source, operators can gradually decrease solar production before the sky darkens depending on weather conditions.”
Over the years, CAISO and other electricity operators have been trying to resolve the obvious limitations the industry faces during major events like eclipses. Francis O’Sullivan, research director at the MIT Energy Initiative, predicts that following the eclipse, there will be an “increased focus on ensuring that we have backup that’s more flexible.” And while the upcoming eclipse will inevitably test the US solar economy, hopefully it will give experts a better understanding of whether the intermittencies will impede solar expansion across the country.
Annette Choi is a freelance journalist based in Los Angeles who covers science and public health issues.
Further reading:
Check our eclipse interactive that shows you what you’ll see, and the time you’ll see it, in your zip code.
Brad Plumer wrote for Vox about the challenge the 2015 solar eclipse posed to solar producers in Europe.

How is it that the disk of the moon (which is actually spiraling into a more distant orbit) is exactly the right size at this moment in geological time to eclipse the sun leaving only the corona visible? The answer is surprising: pure coincidence. It seems to me that the near-miracle of a total eclipse is well worth a little inconvenience in terms of the power grid.

There's nothing remarkable about it. All one has to do is hit the right keys at the right time and the instrument plays itself.
-- Johann Sebastian Bach